Have you tried using plotmath expressions in your axis labels?
I don’t know if that will work, since I’m not sure what the hist3D() function you are using is. The only package loaded in your code is scatterplot3d, but that package doesn’t have a hist3D() function.
It would help a lot if you could turn your question into a small reproducible example that contains all the necessary library() statements and self-contained code that somebody else can run.
What’s even more helpful is if you can run your example through the reprex package before posting. reprex formats the example for sharing, including not only the code but also all the output and plots. That makes it much easier for helpers to understand your problem quickly and suggest solutions without lots of back and forth! 